Demand of high bandwidth and application of high peak-to-average power ratio (PAPR) modulated signals in wireless cellular communication have generated intense interest towards wideband Doherty power amplifier (PA). The benefit of high back-off efficiency from Doherty PA manifest themselves as a prime candidate for the next generation high efficiency power amplifiers for the applications of cellular base stations.
The conventional Doherty PA uses the load modulation concept to maintain high efficiency both at peak and back-off power. However, a conventional load modulation network is only suitable for narrow band operation. Therefore, there is significant demand of Doherty PA which can support wide bandwidth and high efficiency simultaneously to meet the requirements of next generation wireless communications.
However, one of the most important design challenges with the Doherty power amplifier is narrow bandwidth performance at back-off power. Several methods are realized to overcome the narrow band operation at back-off power, among them reduced impedance transformation ratio at back-off power and output compensation stage are prominent. Reduced impedance transformation ratio helps to overcome the problem to some extent but it still shows significant variation of real part of impedance at back-off power, as the frequency moves from its center. Similar characteristics are also observed in case of output impedance compensation network scenario and as a result these techniques end-up with low efficiency at back-off power in practical applications. Therefore, it is very attractive to implement a wideband Doherty power amplifier with minimal variation of real part of impedance at back-off power to maintain high efficiency over wide frequency range.
FIG. 1 shows a related art Doherty power amplifier which includes a 3 dB power divider 101 followed by main 102 and auxiliary 103 power amplifiers and a combining network known as Doherty combiner 104. The main and auxiliary amplifiers are typically biased at class AB and class C mode respectively. Also, output matching networks (OMNs) 105, 106 require at the output of main and auxiliary amplifier to match the optimum load of these PAs into desire final load value. The Doherty combiner works as a load modulation network which provides required load conditions for both peak and back-off power levels. As a result, high efficiency can be maintained over a certain power range for signal with high PAPR; typically, 6-dB PAPR or more. However, due to the inherent narrow band characteristic of the λ/4 impedance inverter ZT 107 at back-off, the conventional Doherty combining network is only suitable for narrow band operation; typically 1-5% relative bandwidth.
FIG. 2A shows the equivalent connection at back-off power level, at which the auxiliary amplifier 103 is at off status showing theoretically infinite output impedance. Therefore, the auxiliary amplifier 103 is disconnected from the Doherty combiner 104, as indicted at a portion 202 in FIG. 2A. The Doherty combiner impedance response (real part) denoted by [Zm,bo] at a portion 203 is shown in FIG. 2B. The nature of the input impedance frequency-dependency of the quarter-wave transmission lines 204 and 205 consisting the Doherty combiner 104 is given by a frequency response curve 207 over the design frequency range 209. Ideally, to design a wide bandwidth efficient operation of Doherty PA, the real part of the impedance [Zm,bo] needs to be frequency independent over the frequency range 209, which should be a flat curve as indicated by a dashed line 208. However, this is very challenging to achieve in practice.
FIG. 3 shows a related art reported in U.S. Pat. No. 9,112,458, in which a wideband Doherty power amplifier is realized by introducing a compensation circuit which is configured to reduce the total quality factor of the wideband quadrature-wave impedance transformer as compared to the quality factor of the quarter-wave impedance transformer. In general, compromising the quality factor of any system usually enhances bandwidth but it generates complications. One of the major consequences is that it increases the loss of the system and specially, in case of power amplifier where efficiency is very crucial, any low quality factor network will introduce efficiency degradation, which will not be suitable for next generation low cost energy efficient wireless systems.
Thus, there is a need for an advanced Doherty power amplifier architecture which has wideband combining network while maintaining high efficiency at back-off power, small form factor and reduced complexity design.